Although Hispanic women in California have 50% lower incidence of breast cancer than non-Hispanic white women, breast cancer is the leading incident cancer in Hispanic women. Yet to date, no published data on breast cancer risk factors in US Hispanics are available, and no analytic study has explored the reasons for the differences in incidence rates between Hispanic whites and non-Hispanic whites. We propose to conduct a population-based case-control study in the San Francisco Bay area among women who identify themselves as Hispanic white, and to assess the associations of breast cancer with migration patterns and acculturation, and with potentially modifiable factors such as physical activity, vitamin D from sunlight exposure and diet, and breast-feeding, for which there are at present limited or inconsistent epidemiologic data available. Hispanic white women aged 3 to 79 years with newly diagnosed breast cancer will be ascertained through two population-based cancer registries. Cases will include all Hispanic women diagnosed with breast cancer between 1995 and 1998. Hispanic white controls will be identified through random-digit dialing and frequency-matched to cases by five-year age group in a ratio of 1.5 controls per case. Trained bilingual and bicultural interviewers will administer a structured questionnaire and measure skin pigmentation for an estimated 535 cases and 802 controls. In home interviews information will be collected on migration patterns, acculturation, physical activity, vitamin D from sunlight exposure and diet, breast feeding, and established breast cancer risk factors such as reproductive and body size characteristics. Cases and controls will be compared with regard to these exposure variables, and unconditional logistic regression will be used to estimate the odds ratios for breast cancer associated with these risk factors, controlling for age and potentially confounding variables. We will also compare the prevalence of risk factors and magnitudes of associations in Hispanics with different migration patterns and levels of acculturation. To the extent possible, we will evaluate the degree to which differences in risk factors explain the difference in incidence rates between Hispanic white and non-Hispanic white women.